Factors in breast milk (antibodies, lymphoid cells and bacterial/viral inhibitory agents) provide passive protection for the infant intestine against intraluminal micro-organisms, toxins and antigens. However, breast milk may also actively stimulate the development of the neonate's own intestinal host defenses. Accordingly, our working hypothesis for this proposal is that breast milk can actively stimulate the accelerated development of normal intestinal host defenses in the newborn including the mucosal immune response to ingested antigens. We reported that intestinal microvillus membranes (MVM) from newborn intestine have biochemical and functional differences compared to MVM from adult intestine. These differences in the "intestinal mucosal barrier" may account for the increased attachment and uptake of antigens noted in the newborn (possibly leading to allergic illness) and for an increase response to bacterial toxins (toxigenic diarrhea). We can stimulate the MVM of newborn intestine to assume biochemical and functional characteristics similar to the MVM from intestine of adults by giving cortisone or thyroxin to pregnant dams or to neonates. We now plan to test whether breast milk or its "growth factors" (hormones, lymphokines or epidermal growth factor) can modify the handling of antigens by neonatal enterocytes and stimulate a normal mucosal immune response to these antigens and whether these factors alter the composition of enterocyte membranes to affect bacterial adherence, toxin response and antigen uptake. Using established techniques, we will determine if colostrum and its lymphoid cells modify the response to ingested antigen in the neonate by examining if systemic priming or tolerance occurs. We will also examine antigen handling and Ia antigen expression on enterocytes in the presence of breast milk and study the role of known colostral growth factors on MVM biochemical composition and enterocyte responses to enterotoxin. We will study the effect of breast feeding on glycoslytransferases and MVM glycoprotein and glycolipid receptors. Finally, using a new human fetal transplant model and organ culture/MVM preparation of surgical specimens of intestines from premature infants, we will determine if breast milk alters the mucosal surface response to enterotoxins, antigen binding/uptake and bacteria adherence and MVM receptor expression (Fc, Sc, Ia, GM1). These studies should help define mucosal barrier defects in newborns and help explain the pathogenesis of neonatal diseases such as necrotizing enterocolitis, toxigenic diarrhea and food allergy.